Detergent manufacturers look to provide low density laundry detergent powders that have improved dissolution profiles at cooler washing temperatures, such as 30° C. or 20° C. Water insoluble materials, most notably zeolite builders, have been removed, or their amount present in the powder has been significantly reduced.
Furthermore, there is also a need to ensure that the environmental profile of the laundry detergent powder is as optimal as possible. This has meant that there is a trend for laundry detergent powder manufacturers to remove phosphate material, such as sodium tripolyphosphate (STPP) from the spray-dried powder.
The main process of preparing low density laundry detergent powder is to spray-dry an aqueous slurry comprising detergent ingredients. Typically, this involves spraying the aqueous slurry into a spray-drying tower that has hot air flowing through that then evaporates the water from the slurry droplets, forming spray-dried powder as the material falls down the tower.
However, the Inventors have found that when material such as zeolite and phosphate are removed from the solid content of the aqueous slurry, the temperature of the resultant spray-dried powder that is formed in the spray-drying zone has a tendency to over-heat and its temperature profile is difficult to control. The Inventors have found that the phenomenon of poor temperature control profile is a specific problem for these low built, highly soluble laundry detergent spray-dried powders and hasn't been observed to any appreciable degree before when spray-drying conventional laundry detergent powders.
The Inventors have found that this problem can be alleviated by running the spray-drying tower under a vacuum. The Inventors have found that by ensuring that the spray-drying zone is under a vacuum, i.e. such that the pressure in the spray-drying zone is negative. This ensures that ambient air is sucked into the spray-drying tower, which in turn provides a much need cooling effect on the spray-dried powder formed therein. The Inventors have found that controlling the
vacuum conditions in the spray-drying zone provides good temperature control of the resultant spray-dried powder.